HYDROGEN-BOND BASICITY OF THIOAMIDES AND THIOUREAS

The thermodynamic hydrogen-bond basicity scale pK(HB) (logarithm of th e formation constant of 4-fluorophenol-base complexes in CCl4) has bee n determined for 18 thiocarbonyl bases, mainly thioamides and thiourea s, and correlated to a spectroscopic basicity scale, the nu(OH) freque ncy shifts of methanol hydrogen bonded to the same basks. We confirm t hat the major site for hydrogen bonding is the sulfur atom and that th e thiocarbonyl bases are always weaker than the corresponding carbonyl bases on the pK(HB) scale. Iminology strongly increases hydrogen-bond basicity and the thiourea iminologue Me(2)NC(Me)=NCSNMe(2) is the mos t basic thiocarbonyl base presently known. The apparently high basicit y of secondary cyclic thioamides and thioureas is discussed.